[lld] 19edd67 - [LLD][ELF] - Make compression level be dependent on -On.

[Georgii Rymar via llvm-commits]

Author: Georgii Rymar
Date: 2019-11-26T11:50:22+03:00
New Revision: 19edd675c6321bc0447f459f427fa67ad46f5e2e

URL: https://github.com/llvm/llvm-project/commit/19edd675c6321bc0447f459f427fa67ad46f5e2e
DIFF: https://github.com/llvm/llvm-project/commit/19edd675c6321bc0447f459f427fa67ad46f5e2e.diff

LOG: [LLD][ELF] - Make compression level be dependent on -On.

Currently LLD always use zlib compression level 6.
This patch changes it to use 1 for -O0, -O1 and 6 for -O2.

It fixes https://bugs.llvm.org/show_bug.cgi?id=44089.

There was also a thread in llvm-dev on this topic:
https://lists.llvm.org/pipermail/llvm-dev/2018-August/125020.html

Here is a table with results of building clang mentioned there:

```
Level   Time            Size
0       0m17.128s       2045081496   Z_NO_COMPRESSION
1       0m31.471s       922618584    Z_BEST_SPEED
2       0m32.659s       903642376
3       0m36.749s       890805856
4       0m41.532s       876697184
5       0m48.383s       862778576
6       1m3.176s        855251640    Z_DEFAULT_COMPRESSION
7       1m15.335s       853755920
8       2m0.561s        852497560
9       2m33.972s       852397408    Z_BEST_COMPRESSION
```

It shows that it is probably not reasonable to use values greater than 6.

Differential revision: https://reviews.llvm.org/D70658

Added: 
    lld/test/ELF/compressed-debug-level.test

Modified: 
    lld/ELF/OutputSections.cpp

Removed: 
    


################################################################################
diff  --git a/lld/ELF/OutputSections.cpp b/lld/ELF/OutputSections.cpp
index cc051dba0e0a..8d328626b85f 100644
--- a/lld/ELF/OutputSections.cpp
+++ b/lld/ELF/OutputSections.cpp
@@ -272,7 +272,12 @@ template <class ELFT> void OutputSection::maybeCompress() {
   // Write section contents to a temporary buffer and compress it.
   std::vector<uint8_t> buf(size);
   writeTo<ELFT>(buf.data());
-  if (Error e = zlib::compress(toStringRef(buf), compressedData))
+  // We chose 1 as the default compression level because it is the fastest. If
+  // -O2 is given, we use level 6 to compress debug info more by ~15%. We found
+  // that level 7 to 9 doesn't make much 
diff erence (~1% more compression) while
+  // they take significant amount of time (~2x), so level 6 seems enough.
+  if (Error e = zlib::compress(toStringRef(buf), compressedData,
+                               config->optimize >= 2 ? 6 : 1))
     fatal("compress failed: " + llvm::toString(std::move(e)));
 
   // Update section headers.

diff  --git a/lld/test/ELF/compressed-debug-level.test b/lld/test/ELF/compressed-debug-level.test
new file mode 100644
index 000000000000..38d7d9016d12
--- /dev/null
+++ b/lld/test/ELF/compressed-debug-level.test
@@ -0,0 +1,38 @@
+# REQUIRES: x86, zlib
+
+# RUN: yaml2obj %s -o %t.o
+
+# RUN: ld.lld %t.o -o %t.default --compress-debug-sections=zlib
+# RUN: llvm-readelf --sections %t.default | FileCheck -check-prefixes=HEADER,LEVEL1 %s
+
+# RUN: ld.lld -O0 %t.o -o %t.O0 --compress-debug-sections=zlib
+# RUN: llvm-readelf --sections %t.O0 | FileCheck -check-prefixes=HEADER,LEVEL1 %s
+# RUN: cmp %t.default %t.O0
+
+# RUN: ld.lld -O1 %t.o -o %t.O1 --compress-debug-sections=zlib
+# RUN: llvm-readelf --sections %t.O1 | FileCheck -check-prefixes=HEADER,LEVEL1 %s
+# RUN: cmp %t.default %t.O1
+
+# RUN: ld.lld -O2 %t.o -o %t.O2 --compress-debug-sections=zlib
+# RUN: llvm-readelf --sections %t.O2 | FileCheck -check-prefixes=HEADER,LEVEL6 %s
+
+## LLD uses zlib compression of level 1 when -O0, -O1 and level 6 when -O2.
+## Here we check how -O flag affects the size of compressed sections produced.
+
+# HEADER: [Nr] Name        Type     Address  Off    Size
+# LEVEL1: [ 1] .debug_info PROGBITS 00000000 000094 00001c
+# LEVEL6: [ 1] .debug_info PROGBITS 00000000 000094 00001b
+
+## A little arbitrary debug section which has a 
diff erent size after
+## applying compression of level 1 and 6.
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS32
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_386
+Sections:
+  - Name:    .debug_info
+    Type:    SHT_PROGBITS
+    Content: '01010201020201020102'